One of the properties of capacitors is its ability to hold a charge when
a voltage is applied to it. The buildup of charges inside the capacitor
generates a voltage across it & in opposition of the voltage that is
driving the incoming charges, effectively resisting to their flow.
This effect of resisting current flow into & out of (an apparent flow
"through") is called reactance & is measured in ohms, the same unit
for resistance. This is because an ohm is a unit of opposition to
electric current, so it makes sense that reactance is also measured in
ohms.
With an alternating signal applied to the capacitor, some charge starts
building up inside the capacitor opposing the flow of current, not
enough to block it completely, so it appears to go through the
capacitor; there's some opposition, but not as much as with a constant
current, which it can block completely when fully charged.
As the frequency (number of times the signal completes a cycle of 0v
-> positive peak -> 0v -> negative peak -> 0v) the charge
that accumulates inside the capacitor gets smaller & smaller, up to
the point where virtually no current is stored & all of the signal
gets apparently through the capacitor.
With an increase in frequency, the capacitive reactance goes down in the
same proportion. This has a more formal definition, given by the
equation:
Xc = 1 / (2 pi f C)
where Xc is the capacitive reactance in ohms, f is the signal frequency & C is the capacitance of the component.
The 2 pi comes from the fact that reactance is actually dependent on the
angular velocity of the incoming signal, but since the 2 pi is constant
& increasing angular velocity leads to higher frecuency, it is
sometimes better to think of reactance in terms of just variable
frequency.
For all practical purposes, capacitive reactance follow the same rules
as resistors when combined in series & parallel. This fact is
particularly useful for understanding most filters, since they often
rely on capacitive reactance as part of a voltage divider.
